The present invention relates to a demodulation structure and a method for downconverting and demodulating a modulated digital signal.
Demodulation structures and methods for downconverting and demodulating a digitally modulated signal are known from and used in many different applications, in which signals are transmitted from a transmitting apparatus to a receiving apparatus. The transmission can be either wireless over an air interface or through a wired connection. Of course, the chosen structure and method for downconverting and demodulating a transmitted digitally modulated signal depends essentially on the modulation scheme used on the transmitting side and the transmission medium.
FIG. 10 shows an example for a demodulation structure for downconverting and demodulating a digitally modulated signal S0. The shown demodulation structure is e.g. used for downconverting and demodulating a high frequency signal (RF signal) received in a mobile terminal of a wireless telecommunication system, such as the GSM or the UMTS system. Hereby, a transmitted RF signal is received by an antenna of the mobile terminal and supplied to a RF receiver. The digital RF signal is then supplied to a demodulation structure as e.g. shown in FIG. 10. Another possibility could be to downconvert the RF signal into an intermediate frequency band, whereafter the intermediate signal is further downconverted and demodulated in a structure as shown in FIG. 10.
In wireless telecommunication, usually an I/Q-modulation is used, in which the modulation states can be represented in an I/Q-diagram. The horizontal axis hereby represents the I part, i.e. the part of the signal component which is in phase to the carrier signal, and the vertical axis represents the Q part, i.e. the quadrature component perpendicular to the phase of the carrier signal. The digital information signal is thus transmitted by means of the phase of the carrier signal, whereby the phase of the carrier signal is switched between the different discrete states depending on the symbol to be transmitted. This so-called phase shift key modulation (PSK modulation) is used in many different application areas, very often in combination with amplitude shift keying. The GSM e.g. uses a GMSK modulation (Gaussian minimum shift keying modulation).
Demodulation and downconversion of correspondingly I/Q-modulated signals is e.g. done by analog demodulation structures as shown in FIG. 10 comprising a local oscillator 101, two mixers 102, 103, a 90 degree phase shifter 104, two lowpass filters 105, 106 and two analog-to-digital-converters 107, 108. A local oscillator signal generated in the local oscillator 101 is directly supplied to the first mixer 102 and indirectly supplied to the second mixer 103 via the 90 degree phase shifter 104. In the mixers 102 and 103, the digitally modulated signal S0 is mixed with the corresponding local oscillator signal and the phase shifter local oscillator signal, respectively, so that two separate output streams, one for the in phase (I) part and one for the quadrature phase (Q) part are generated. After further processing in the lowpass filters 105 and 106 and the analog-to-digital converters 107 and 108, the in phase part and the quadrature phase part are obtained in parallel as digital signals SI and SQ.
The disadvantages of this conventional approach are that the necessary 90 degrees phase shifter is an analog device which is inherently frequency dependent and thus limiting the frequency range of the entire demodulation structure due to the increasing phase and amplitude imbalances. Further, since two parallel output streams are generated, two mixers, two lowpass filters and two A/D-converters are required, so that the entire demodulating structure requires many parts and becomes therefore expensive.
The object of the present invention is therefore to provide a demodulation structure and a method for downconverting and demodulating a digitally modulated signal, which is simple in construction but still effective in operation.
This object is achieved by a demodulation structure for downconverting and demodulating a digitally modulated signal according to claim 1, with a local oscillator means for providing a local oscillator signal, a mixer means for mixing said local oscillator signal and said digitally modulated signal in order to obtain a mixed signal, a lowpass filter means for lowpass filtering said mixer signal from said mixer means, and an analog-to-digital converting means for converting the filtered signal from said lowpass filter means into a downconverted and demodulated digital signal, whereby said local oscillator signal is set in respect to said modulated digital signal so that said downconverted and demodulated digital signal output from said analog-to-digital converting means comprises two serially arranged information parts. The above object is further achieved by a method for downconverting and demodulating a digitally modulated signal according to claim 11.
Compared to the state of the art, the demodulation structure and method are particularly advantageous since the two information parts are obtained serially instead of parallel which allows a very simple construction of the respective devices. This is particularly advantageous in case of an application of the present invention in mobile or portable terminals, like mobile terminals for a wireless telecommunication system, since the overall weight can be significantly reduced. Further, the usually analog phase shifters of the prior art devices can be omitted and therefore the amplitude and the phase imbalances can be essentially reduced. Further, the present invention has the potential to provide demodulation structures and methods with a larger bandwidth of operation as the prior art.
Advantageously, the digitally modulated signal is I/Q-modulated and the two serially arranged information parts comprised in the downconverted and demodulated digital signal are an I-part and a Q-part of the I/Q-modulated digital signal.
Further advantageously, the digitally modulated signal is modulated in a signal band having a center frequency and said local oscillator signal has a center frequency, which is, in respect to the center frequency of the signal band, offset by half of the signal bandwidth of the modulated digital signal.
Alternatively, the local oscillator signal is modulated with at least two modulation states having different phases during the symbol period of the digitally modulated signal. In this case, the two different modulation states may have the same magnitude and a 90 degree phase shift in respect to each other. The demodulation structure according to the present alternative of the present invention may further comprise a modulation control means for supplying a modulation signal to the local oscillator means in order to internally modulate the local oscillator signal with the two modulation states. Alternatively, the demodulation structure of the present alternative of the demodulation structure according to the present invention may comprise an analog circuit means for modulating the local oscillator signal from the local oscillator means with the two modulation states and outputting a modulated local oscillator signal to the mixer means. Hereby, the analog circuit means may comprise a switch means which can be switched between a first branch having a phase shift means and a second branch having no phase shift means, whereby the switch means is switched by means of a control signal with a frequency of at least two times the symbol frequency of the digitally modulated signal.
The present alternative of the demodulation structure according to the present invention may further advantageously comprise a bandpass filter for bandpass filtering the modulated local oscillator signal. Advantageously, the bandpass filter has a center frequency corresponding to the center frequency and a bandwidth corresponding to the bandwidth of the signal band of the digitally modulated signal.
Advantageous features of the method for downconverting and demodulating a modulated digital signal according to the present invention are defined in the subclaims 12 to 20.